This invention relates to Si.sub.3 N.sub.4 ceramic articles, and more particularly relates to such articles in monolithic form having a lower density outer layer, and to a method for producing them.
Current interest in Si.sub.3 N.sub.4 ceramics reflects a recognition that their good-to-excellent high temperature mechanical strength, thermal shock resistance, and chemical inertness make these materials excellent candidates for structural applications at temperatures above those tolerated by superalloy materials.
An outstanding example of such a structural application is the gas turbine engine, wherein Si.sub.3 N.sub.4 is being considered for rotor and stator vanes, shrouds and sealing rings.
Due to very stringent tolerance requirements between the rotor vanes and the surrounding sealing ring, a seal with an abradable inner layer is being considered, wherein during engine start-up the rotor vanes would seat themselves against the seal by thermal expansion into the abradable layer.
Techniques are known for the production of high density (and thus high strength) Si.sub.3 N.sub.4 bodies by hot pressing and by pressureless sintering. U.S. Pat. No. 4,073,845, issued to Buljan et al. on Feb. 14, 1978, describes the pressureless sintering of partly crystalline, partly amorphous starting material to obtain sintered densities previously obtained only by hot pressing.
Si.sub.3 N.sub.4 bodies having density differences between inner and outer portions may be produced by seperately forming elements of such different densities, and bonding the elements into a composite structure. For example, U.S. Pat. No. 3,885,294, issued May 27, 1975, forms a first article portion by nitridation, and then bonds a higher density layer to it by hot pressing. Such techniques tend to be complex and time-consuming.
Another technique for achieving such different densities is to carry out nitridation of silicon powder in-situ on a Si.sub.3 N.sub.4 substrate, as reported by Brennan et al., Government Contract Report NAS3-19731, Oct. 25, 1977.